1. Technical Field
Apparatuses and methods consistent with the present invention relate to a laser projection system reducing speckle noise, and more particularly, to a composite scanning unit sequentially scanning diffraction beams of different orders on a screen to reduce speckle noise and a laser projection system employing the same.
2. Description of the Related Art
A projection system modulates a plurality of monochromatic beams using image information, and then magnifies and projects the modulated monochromatic beams onto a screen so as to display a color image. The projection system may adopt various types of light sources. A laser light source emits a laser beam having a greater intensity and a higher luminance than other light sources. Thus, the laser light source can provide a clear image having a high contrast and, thus, is preferred. However, a laser beam has is highly coherent, which causes speckle noise. A speckle is a mottled pattern generated by a peculiar fluctuation of a laser beam incident on a screen due to surface roughness of the screen and remarkably deteriorates image quality.
Methods of reducing speckle noise are generally based on a formation of time varying speckle patterns in a detecting system. The speckle noise may be improved, from the point of view of a detector, e.g., eyes of a human, by time averaging. Thus, an image frame having N uncorrelated speckle patterns must be formed to reduce the speckle noise. A speckle contrast C is defined as a dispersion over an average of light intensities indicated by speckle, i.e., “C=σI/<I>.” Thus, if the image frame has N uncorrelated speckle patterns, the speckle contrast C is reduced to 1/√{square root over (N)}. As a result, it is important how N uncorrelated speckle patterns are to be formed. For this, a method of producing a spatial diversity of a laser beam, a method of producing a time diversity of the laser beam, a method of increasing a bandwidth, a method of diverging a phase, etc. have been suggested.
A solution to the speckle noise is limited by a structure and technical requirements of a used optical system. For example, if a variable diffractive grating device, such as a grating light valve (GLV), is used as a light modulation device, an image contrast is very sensitive to a bandwidth of the wavelength of light incident on the variable diffractive grating device, and brightness of pixels on a screen is affected by a relative variation of a variable ribbon of the GLV generating a phase difference. The phase difference is a function of the wavelength. If the bandwidth of the wavelength is increased, the image contrast is worsened. Therefore, if the bandwidth is increased enough to reduce speckle noise, the image contrast is remarkably deteriorated. In other words, a projection system using such a variable diffractive grating device must adopt a highly monochromatic laser source and cannot employ a method of realizing a reduction in speckle noise by increasing a bandwidth of the wavelength of light.